As the first step in fertilization, sperm of both sea urchins and mammals exhibit species specific adhesion to the glycoprotein envelope covering the egg plasma membrane. After the attachment forms the sperm penetrates the egg envelope and the plasma membranes of the two gametes fuse. Bindin is the protein mediating the attachment of sperm to sea urchin eggs. We now believe that bindin is also active in promoting the fusion of gamete membranes. During the next year we will: 1) continue to perfect simple purification procedures for bindins from different species, 2) continue the amino acid sequence analysis of bindin, 3) characterize the egg surface receptor for bindin, 4) investigate the attachments between the egg vitelline layer, the plasma membrane and the cortical actin filaments, 5) isolate and characterize the egg cortex after fertilization and throughout the cell cycle, 6) characterize the proteins of the sperm plasma membrane, especially the major components of 86 and 64K daltons which we have recently discovered, and 7) continue working on the hypothesis that bindin is a fusigenic protein. Sea urchin sperm bindin is the first animal sperm protein to be isolated and shown to interact in a specific manner with an egg surface glyoprotein receptor. The bindin receptor is the first egg surface component to be isolated that possesses the criteria of a "sperm receptor". The study of sperm bindin and its egg surface receptor may lead to the development of novel non-hormonal strategies of contraception in higher organisms. The study of the changes occuring to the egg cortex during cell cycle may also extend our knowledge of the components regulating the cell cycle and especially the mechanism of cytokinesis.